The Her-2/neu oncogene is expressed in approximately one third of breast cancers. It has both an intracellular and extracellular component. Her-2/neu overexpression correlates with disease -free and overall survival in patients with breast cancer. It also correlates with sensitivity and response to various types of chemotherapy used to treat breast cancer. The extracellular domain (ECD) is found in the serum of patients with breast cancer and correlates highly with the expression of Her-2/neu in the breast tumor tissue. If it can be shown that rising levels of serum Her-2ECD levels can predict response , then therapy which is currently available against the Her-2 domain can potentially be directed against it as a way of prolonging survival in patients with breast cancer. The purpose of this study is to evaluate changes in serum Her-2 ECD after surgery and after adjuvant chemotherapy for primary breast cancer. The purpose is also to evaluate whether serum Her-2 ECD antigen levels predict relapse in patients who have undergone definitive treatment for cancer of the breast. The study will also evaluate whether changes in serum Her-2 ECD antigen levels correlate with clinical status in patients receiving therapy for advanced breast cancer.